1. Field of the Invention
The present invention relates to a secondary battery system for a hybrid system, an electric power generating system, and the like on which a secondary battery is mounted.
2. Description of the Related Art
For automobiles, rail vehicles, and the like, there has been known a hybrid system in which a rechargeable battery such as a lead acid battery, a nickel metal hydride battery, and a lithium battery is mounted, electric power obtained by a regeneration brake is charged to the rechargeable battery, and the electric power is utilized at the time of acceleration or the like, to thereby enhance fuel efficiency.
In addition, for an electric power generating system which utilizes natural energy, such as wind power generation and photovoltaic power generation, in order to convert largely varying natural energy into electricity, load leveling using a secondary battery system for storing electricity has been required in the case of connection with an electric power transmission system.
In the vehicles such as automobiles which adopt the hybrid system as described above or the electric power generating system including the secondary battery for storing electricity as described above, it is extremely important to know the deterioration state of the used secondary battery in order to maintain the energy efficiency of the entire system.
Therefore, technologies of presuming the deterioration state of the secondary battery have been proposed in the past. For example, Japanese Patent Laid-Open Publication No. 2003-129927 (Patent Document 1) discloses a method of measuring, for each engine start, a current flowing in the secondary battery and an open circuit voltage to thereby calculate a direct current resistance, and then calculating a state of health (SOH) of the secondary battery on the basis of the history of the direct current resistance.
Japanese Patent Laid-Open Publication No. 2007-55450 (Patent Document 2) discloses a technology in which a driving history including charge and discharge data of the secondary battery is transmitted to the outside, to thereby calculate, by a driving center, the deterioration state of the secondary battery mounted on the vehicle, and if the difference in calculation results of the deterioration state between the vehicle and the driving center exceeds a reference value, the driving center gives a modification instruction.
Further, Japanese Patent Laid-Open Publication No. 08-43505 (Patent Document 3) discloses that the remaining capacity of the secondary battery is calculated on the basis of the ratio of a direct current resistance at the time of discharge of the secondary battery to a direct current resistance at the time of charge thereof.
However, the direct current resistance of the secondary battery does not deteriorate at a constant rate, but is affected by a battery usage history and an environment state such as a total charge and discharge amount, a maximum current value, and a history of temperature at which the secondary battery has been stored.
Further, it is known that, if an excessively large current is repeatedly charged to and discharged from the secondary battery capacity in a short period of time, the life of the secondary battery considerably deteriorates.
Therefore, in the secondary battery system for the hybrid system, the wind power generating system, the photovoltaic power generating system, and the like which use the secondary battery, in order to suppress the deterioration of the secondary battery to the minimum, normally, it is common practice to limit the charge and discharge amount in a short period of time to, for example, approximately a few percent of the secondary battery capacity.
However, in the cases where a driving situation is rapidly changed, an abnormality in a charge and discharge management system occurs, and a secondary battery having a capacity which is excessively small compared with the used system is designed, a large current may be repeatedly charged to and discharged from the secondary battery capacity frequently and continuously for a long period of time. In such a case, there is fear that the life of the secondary battery considerably deteriorates.
This property is particularly remarkable when the secondary battery is a lithium ion battery.
The inventors of the present invention figured out the cause of this property as follows. Specifically, when a large current is charged to and discharged from the secondary battery capacity, separately from an irreversible increase in direct current resistance which decides an essential life of the secondary battery, even if the direct current resistance of the secondary battery temporarily increases, the direct current resistance decreases over time to recover. Further, if the charge and discharge is repeated again before the direct current resistance of the secondary battery which has temporarily increased sufficiently decreases, the life of the secondary battery considerably deteriorates.
That is, it was found out that, even if a large current is charged to and discharged from the secondary battery capacity to temporarily increase the direct current resistance of the secondary battery, if this direct current resistance of the secondary battery decreases to a certain degree, it is possible to effectively prevent the life of the secondary battery from considerably deteriorating, even if the charge and discharge is performed again.
However, according to the method disclosed in Japanese Patent Laid-Open Publication No. 2003-129927 (Patent Document 1), it is not possible to discriminate between the deterioration due to the transitional and temporary increase in direct current resistance as described above and the continuous deterioration over time, and hence the deterioration state of the secondary battery cannot be accurately detected. As a result, there is fear that the temporary deterioration of the direct current resistance is erroneously determined as the continuous deterioration over time, and that the life of the secondary battery is rapidly shortened by performing the charge and discharge before the recovery from the transitional increase in direct current resistance.
According to the method disclosed in Japanese Patent Laid-Open Publication No. 2007-55450 (Patent Document 2), a storage device for calculating the deterioration state on the basis of the driving history including the charge and discharge data and an arithmetic device for this calculation need to be provided to both of the vehicle and the driving center, and communication means between the vehicle and the driving center is also required. Accordingly, the system is high in cost and complicated. Further, the degree of progress of the deterioration is estimated on the basis of the rate of increase with respect to the direct current resistance of the secondary battery in its initial state. Accordingly, the temporary increase in direct current resistance of the secondary battery, which is caused by charging and discharging a large current as described above, is not considered.
In addition, Japanese Patent Laid-Open Publication No. 08-43505 (Patent Document 3) discloses that the remaining capacity of the secondary battery is calculated on the basis of the ratio of the direct current resistance at the time of the discharge of the secondary battery to the direct current resistance at the time of the charge thereof. As shown in FIG. 4, the ratio of the discharge direct current resistance to the charge direct current resistance is constant independently of the deterioration state. Then, Japanese Patent Laid-Open Publication No. 08-43505 (Patent Document 3) evidently premises the use within a range in which a discharge current is a low current, and thus focuses on detecting the remaining capacity of the secondary battery on the basis of the ratio of the direct current resistance at the time of the discharge to the direct current resistance at the time of the charge. Accordingly, it is obvious that Japanese Patent Laid-Open Publication No. 08-43505 (Patent Document 3) does not assume the charge and discharge with a large current concerning a problem to be solved by the present invention.
In view of the above, the present invention has an object to simply detect, in a secondary battery system used in a hybrid system, a wind power generating system, a photovoltaic power generating system, and the like, the situation where a direct current resistance thereof transitionally (temporarily) increases, on the basis of the usage situation of a secondary battery, to thereby accurately calculate the deterioration state of the secondary battery and thus adopt an appropriate using method, so that the life of the secondary battery is extended.